The overall goal of this application is to understand the subcellular mechanisms of cardiac protection by calcium preconditioning against calcium overload, which is commonly observed during post ischemic reperfusion. The recently discovered phenomenon of Ca++ preconditioning i.e. multiple l minute Ca++ depletion and 5 minutes Ca++ repletion, confers strong protection if these hearts are subjected to the Ca++ overload or ischemia/reperfusion. The preliminary data suggest that the reduction of cellular injury is mediated by adenosine and calcium triggered mechanisms. The mechanisms underlying the unique protection by Ca++ preconditioning remain to be elucidated. The specific aims of this proposal are centered around three major triggers of Ca++ preconditioning i.e. release of adenosine, catecholamine and elevation of intracellular Ca++ and will address the following questions. l) How does endogenous adenosine released during calcium preconditioning protect the heart against severe Ca++ overload injury? 2) Does adenosine released during Ca++ preconditioning provide protection as a result of its antiadrenergic effects? 3) Does catecholamine released during Ca++ preconditioning protect against the Ca++ overload by activating alpha1 adrenergic receptors? 4) Do adenosine and Ca++ elevation during Ca++ preconditioning activate K+ ATP channel? 5) Do Ca++ increase or other mediators released during Ca++ preconditioning activate protein kinase C to provide protection via different intracellular signaling pathways? 6) Do interventions that raise intracellular Ca++ mimic Ca++ preconditioning? 7) Can mild stress such as oxidants/oxygen radicals induce protection similar to Ca++ preconditioning? 8) Finally, does Ca++ preconditioning protect against the ischemia/reperfusion injury? The experimental protocols will use pharmacological approaches and measurement of several different variables, e.g. measurement of intracellular Ca++ in myocytes, cardiac function, nucleotides, catecholamine and enzyme release, electron microscopy, immunolocalization of protein kinase C isoforms, and other biochemical parameters of cell injury. These studies will provide insights into the potential involvement of adenosine and calcium-dependent pathways in the protection of cell necrosis that typically occurs during Ca++ overload. This investigation will have important implications for l) understanding mechanisms of the cellular injury induced by Ca++ overload which occurs in pathological conditions and 2) how the heart protects itself from Ca++ overload, 3) the designing of therapeutic interventions based on the unique endogenous protection elicited by calcium preconditioning against Ca+ + overload and ischemia/reperfusion injury in trauma patients or during thrombolytic therapy, percutaneous transluminal angioplasty and coronary bypass grafting.